Conceptually, a physical file system architecture includes system meta-data, Msys, object meta-data, Mobject, and object data. The Msys data describes the entire file system. For example, Msys, includes system parameters, an i-node bitmap, a block bitmap and other file system-dependent information.
The Mobject data describes a file or directory. In an example physical file system, a particular Mobject segment includes the file i-node and direct and indirect blocks. The i-node includes information such as the name of the file, access rights, update times, pointers to data blocks, and pointers to indirect, double indirect, and triple indirect blocks. The indirect blocks also contain pointers to data blocks. The file data is the data referenced and manipulated by one or more client application programs.
A distributed file system is generally characterized by multiple storage servers that store and serve data to client applications. The storage servers and systems that host the client applications are connected to a network. A locking mechanism provides data consistency as between multiple clients. For example, when a client is writing to a file, the i-node is locked to prevent another client from manipulating the same file. Any operations that result in allocating or de-allocating i-node or data blocks cause the associated bitmap areas to be locked.
In some distributed file systems, the locking mechanism is provided by a meta-data server. The meta-data server manages some implementation-dependent combination of Msys and Mobject, and separate storage servers provide storage for file data. A lease mechanism is often used for locking objects. Leases are managed and granted by the meta-data server. A lease for an object is for a selected period of time and is granted in response to a lease request from a client. The type of lease granted to one client determines whether a lease will be granted to another client. For example, if one client is granted a write lease, no other client will be granted a write lease until the first write lease expires.
The separation of storage servers from the meta-data server creates the potential for timing-related problems in servicing write leases. Since the storage server is not involved in the management of leases, it does not have the information necessary to enforce data consistency by providing or denying file access requested from a client. However, if too much information is shared between the meta-data server and the storage servers, the benefits of scalability provided by the separation of functions would be adversely impacted.
A system and method that address the aforementioned problems, as well as other related problems, are therefore desirable.